The present invention pertains to a vacuum chuck for holding thin sheet material such as a glass substrate, and to an apparatus for forming a substantially uniform coating of a substance on a thin sheet material such as a glass substrate.
Thin substrates are used to manufacture numerous items and often require high-temperature processing during the manufacturing. By way of example, thin glass solar cell wafers are utilized for satellite power supply applications and frequently are formed by coating one or more substances on the glass substrates in a high-temperature processing zone. For example, an amorphous silicon coating might be formed on a 3 to 5 mil thick glass wafer in a chemical vapor deposition process at a high temperature. The coating must be uniform over the surface of the wafer, or the resulting coated glass wafer is not usable for its intended purpose.
Present processing techniques for forming a coating on thin substrates include conveying the substrates slowly through a series of processing zones, with significantly different temperatures in each zone. By way of example, the series of processing zones may have a total length in the order of ten feet and may be divided by baffles into three or more zones, with the first zone being heated to a temperature in the order of 200.degree. C. the second zone heated to a temperature in the order of about 600.degree. C. and the third zone heated to a temperature in the order of about 200.degree. C. In some processes a different substance is coated on the substrates in each zone, while in other processes a substance is coated on the substrates in only a single one of the zones. The substrates might be conveyed through these zones at speeds as slow as one foot per minute or slower. Each substrate might have a length of about 12 inches or longer. Consequently, as a substrate moves from one zone to the next, for a significant period of time a portion of the substrate is in the higher temperature zone, while the remainder of the substrate is in the lower temperature zone. During this time a significant temperature gradient exists over the length of the substrate. This temperature gradient can result in the thin substrate warping. During the deposition of substances onto the substrate, such warping can cause non-uniform layer growth across the substrate. The resulting coated substrate is unsatisfactory due to this non-uniform coating.
Vacuum chucks with rubber valves have been used in some room temperatures processing techniques. However, the rubber valves are not able to withstand the high temperatures and corrosive atmospheres encountered in chemical vapor deposition as utilized in many manufacturing processes.